SNAP receptor (SNARE) proteins catalyze biological membrane fusion and contribute to the fidelity of intracellular membrane traffic - they are perhaps the most fundamental determinant of an organelle's trafficking in the cell. The yeast prenylated SNARE ykt6p appears to have multiple functions in the constitutive secretory pathway. Unexpectedly, our results demonstrate that mammalian ykt6 does not have the characteristics of a SNARE for constitutive secretion, as it is expressed predominantly in neurons. Furthermore, membrane bound rat ykt6 localizes to unique cytoplasmic vesicular structures that do not overlap appreciably with recognized organelles of the secretory or endosomal systems. This proposal aims to dramatically advance our understanding of neuronal ykt6 and the specialized transport process(es) in which it participates. Our guiding hypothesis is that rat ykt6 is involved in the trafficking of a highly specialized transport organelle containing cargo that is important for aspects of neuronal function. The specific aims are: 1. Test the hypothesis that ykt6 interacts with a unique subset of neuronal SNAREs and other membrane and cytosolic proteins. 2. Test the hypothesis that ykt6 is localized to a specialized transport compartment containing cargo relevant to neuronal function. 3. Test the hypothesis that the NT domain targets ykt6 to its specialized location by binding to a specific membrane receptor. 4. Test the hypothesis that cytosolic ykt6 SNARE interactions are autoinhibited by cooperation between the NT domain and prenyl group(s). These studies will dramatically advance our understanding of membrane trafficking in neurons, and highlight underlying processes that may contribute to human disease. Many of the SNARE regulatory principles discovered in this work will be applicable to SNARE function at multiple membrane transport steps. Importantly, ykt6 is a prenyl protein that is predicted to be modified by protein farnesyltransferase. There is currently little information about the function of farnesylated proteins not involved in cell proliferation. Since many of the most promising new cancer treatments use drugs to inhibit protein farnesyltransferase, it is essential to understand the major metabolic pathways affected when protein farnesylation is stopped. To understand and potentially compensate for the unintended consequences of these drugs in neurons, it is essential to understand the function of the ykt6 membrane transport pathway.